One-Dimensional Response of a Borehole Station during the 2005 West off Fukuoka Prefecture Earthquake: Observation and Simulation

Abstract eng:
The objective of this paper is to present a one-dimensional response of a soil column at a borehole station during the 2005 west off Fukuoka prefecture, Japan, earthquake. The borehole station is located in Fukuoka City where the sediment’s thickness reaches 56 meters. First, we have confirmed that according to the rupture process the major axis is north 32 degrees east by calculating the observed energy distribution at several stations around the epicenter. Then we have computed, in the major and minor axes, surface-to-borehole spectral ratios using the S-wave portion of the main shock and 12 aftershocks. As for the aftershocks, the fundamental resonant peaks lie in the frequency range of 1.5 to 1.9 Hz, and are quite stable. As for the main shock, the resonant peak is 1.3 Hz showing a nonlinear effect. To simulate the main shock, we have first inverted by genetic algorithm S-waves velocities and damping coefficients of the soil column as well as incident angle of the upcoming wave using Thomson-Haskell propagator matrix for SH and P-SV waves. Inverted values of S-waves velocities are consistent with the P-S logging provided by CTI Engineering and inverted incident angle is also consistent with observed particle orbit in the vertical plane. Simulations at the borehole station show a good agreement with observations.

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